1. Technical Field
The invention relates to processes for de-oxidizing or cleaning surfaces such as aluminum and then forming an adhesive oxide layer thereon suitable for bonding thereto.
2. Background Art
Many techniques are known in the art for treating aluminum surfaces, as disclosed in U.S. Pat. Nos. 3,834,998; 4,085,012; 4,127,451; 4,397,716; 4,623,591; 4,624,752; 4,647,346; 4,793,903 and 4,853,093. Such techniques are generally characterized by separate chemical baths for performing de-oxidation or for performing anodization. For example, U.S. Pat. No. 4,793,903 discloses an electrolytic process for de-oxidizing aluminum surfaces in a first chemical bath containing a phosphoric acid solution which is followed by a rinse step prior to a subsequent anodization step. The subsequent anodization step deposits a strongly adherent aluminum oxide layer suitable for bonding, and is carried out in a second chemical bath containing a phosphoric acid solution as disclosed in U.S. Pat. Nos. 4,085,012 and 4,127,451. The second chemical bath is free of the contaminants removed in the de-oxidizing bath. The intervening rinse step prevents any transfer of such contaminants between the two baths via the workpiece.
All of the foregoing is distinguished from electrolytic polishing techniques of the type disclosed in U.S. Pat. No. 2,708,655, in which an aluminum workpiece is polished by first de-oxidizing it in a chemical bath containing phosphoric and sulfuric acids and ethylene glycol mono ethyl ether, after which it is electrolytically polished in the same bath and rinsed in a wetting solution. This process deposits a thin non-adherent aluminum oxide layer, which is removed by immersion in another chemical bath which does not attack the now-polished surface prior to any anodizing steps. Anodization may occur in yet another chemical bath comprising either a chromic or sulfuric acid solution.
A major disadvantage of the phosphoric acid electrolytic de-oxidizing and anodization process of U.S. Pat. Nos. 4,793,903, 4,085,012 and 4,127,451 is its high capital cost. Specifically, as many as four chemical baths are required: the phosphoric acid solution for de-oxidizing, the intervening rinse bath, the phosphoric acid solution for anodizing and a final rinse bath. Consider for example a process in which each bath contains 13,000 gallons in a stainless steel container. The two rinse tanks would have to be disposed of periodically as they accumulate acid rinsed off many successive work pieces. Disposal of two 13,000 gallon chemical rinse tanks from the Los Angeles, California metropolitan area costs approximately $39,000. Moreover, each of the two acid baths requires a 13,000 gallon stainless steel container and related equipment including a rectifier for the electrolytic source, a temperature control unit, and agitation systems.
Unfortunately, it has seemed that, in view of the requirement to keep the anodization bath free of the contaminants created during the de-oxidation step, no reduction in the number of chemical baths is possible for practical application.
Accordingly, it is an object of the present invention to provide a process for electrolytically de-oxidizing and anodizing a surface such as an aluminum surface using a smaller number of chemical baths, without compromising the integrity of the anodization step through the introduction of contaminants from the de-oxidation step.